Conventional computer systems include a processor coupled to a variety of memory devices, including read-only memories ("ROMs") which traditionally store instructions for the processor, and a system memory to which the processor may write data and from which the processor may read data. The processor may also communicate with an external cache memory, which is generally a static random access memory ("SRAM"). The processor also communicates with input devices, output devices, and data storage devices.
Processors generally operate at a relatively high speed. Processors such as the Pentium.RTM. and Pentium Pro.RTM. microprocessors are currently available that operate at clock speeds of at least 200 MHz. However, the remaining components of the computer system, with the exception of SRAM cache memory, are not capable of operating at the speed of the processor. For this reason, the system memory devices, as well as the input devices, output devices, and data storage devices, are not coupled directly to the processor bus. Instead, the system memory devices are generally coupled to the processor bus through a memory controller, and the input devices, output devices, and data storage devices are coupled to the processor bus through a bus bridge. The memory controller allows the system memory devices to operate at a clock frequency that is substantially lower than the clock frequency of the processor. Similarly, the bus bridge allows the input devices, output devices, and data storage devices to operate at frequency that is a substantially lower than the clock frequency of the processor. Currently, for example, a processor having a 200 MHz clock frequency may be mounted on a mother board having a 66 MHz clock frequency for controlling the system memory devices and other components.
Access to system memory is a frequent operation for the processor. The time required for the processor, operating, for example, at 200 MHz, to read data from or write data to a system memory device operating at, for example, 66 MHz, greatly slows the rate at which the processor is able to accomplish its operations. Thus, much effort has been devoted to increasing the operating speed of system memory devices.
System memory devices are generally dynamic random access memories ("DRAMs"). Initially, DRAMs were asynchronous and thus did not operate at even the clock speed of the motherboard. In fact, access to asynchronous DRAMs often required that wait states be generated to halt the processor until the DRAM had completed a memory transfer. However, the operating speed of asynchronous DRAMs was successfully increased through such innovations as burst and page mode DRAMs, which did not require that an address be provided to the DRAM for each memory access. More recently, synchronous dynamic random access memories ("SDRAMs") have been developed to allow the pipelined transfer of data at the clock speed of the motherboard. However, even SDRAMs are typically incapable of operating at the clock speed of currently available processors. Thus, SDRAMs cannot be connected directly to the processor bus, but instead must interface with the processor bus through a memory controller, bus bridge, or similar device. The disparity between the operating speed of the processor and the operating speed of SDRAMs continues to limit the speed at which processors may complete operations requiring access to system memory.
A solution to this operating speed disparity has been proposed in the form of a computer architecture known as "SyncLink." In the SyncLink architecture, the system memory may be coupled to the processor directly through the processor bus. Rather than requiring that separate address and control signals be provided to the system memory, SyncLink memory devices receive command packets that include both control and address information. The SyncLink memory device then outputs or receives data on a data bus that is coupled directly to the data bus portion of the processor bus.
An example of a computer system 10 using the SyncLink architecture is shown in FIG. 1. The computer system 10 includes a processor 12 having a processor bus 14 coupled to three packetized dynamic random access memory or SyncLink DRAMs ("SLDRAM") devices 16a-c. The computer system 10 also includes one or more input devices 20, such as a keypad or a mouse, coupled to the processor 12 through a bus bridge 22 and an expansion bus 24, such as an industry standard architecture ("ISA") bus or a Peripheral component interconnect ("PCI") bus. The input devices 20 allow an operator or an electronic device to input data to the computer system 10. One or more output devices 30 are coupled to the processor 12 to display or otherwise output data generated by the processor 12. The output devices 30 are coupled to the processor 12 through the expansion bus 24, bus bridge 22 and processor bus 14. Examples of output devices 24 include printers and a video display units. One or more data storage devices 38 are coupled to the processor 12 through the processor bus 14, bus bridge 22, and expansion bus 24 to store data in or retrieve data from storage media (not shown). Examples of storage devices 38 and storage media include fixed disk drives floppy disk drives, tape cassettes and compact-disk read-only memory drives.
In operation, the processor 12 communicates with the memory devices 16a-c via the processor bus 14 by sending the memory devices 16a-c command packets that contain both control and address information. Data is coupled between the processor 12 and the memory devices 16a-c, through a data bus portion of the processor bus 14. Although all the memory devices 16a-c are coupled to the same conductors of the processor bus 14, only one memory device 16a-c at a time reads or writes data, thus avoiding bus contention on the processor bus 14. Bus contention is avoided by each of the memory devices 16a-c on the bus bridge 22 having a unique identifier, and the command packet contains an identifying code that selects only one of these components.
The computer system 10 also includes a number of other components and signal lines which have been omitted from FIG. 1 in the interests of brevity. For example, as explained below, the memory devices 16a-c also receive an external clock signal CKEXT to provide internal timing signals, a data clock signal DCLK clocking data into or out of the memory device 16, and a FLAG signal signifying the start of a command packet.
One of the memory devices 16a is shown in block diagram form in FIG. 2. The memory device 16a includes a clock generator circuit 40 that receives a master clock signal CKEXT and generates an internal clock signal CKINT and a large number of other clock and timing signals to control the timing of various operations in the memory device 16. The memory device 16 also includes a command buffer 46 and an address capture circuit 48 which receive an internal clock signal CKINT, a command packet CA0-CA9 on a 10-bit command bus 50, and a FLAG signal on line 52. The memory controller (not shown) or other device normally transmits the command packet CA0-CA9 to the memory device 16a in synchronism with the external clock signal CKEXT. As explained above, the command packet contains control and address information for each memory transfer, and the FLAG signal identifies the start of a command packet which may include more than one 10-bit packet word. In fact, a command packet is generally in the form of a sequence of 10-bit packet words on the 10-bit command bus 50. The command buffer 46 receives the command packet from the bus 50, and compares at least a portion of the command packet to identifying data from an ID register 56 to determine if the command packet is directed to the memory device 16a or some other memory device 16b, c. If the command buffer 46 determines that the command packet is directed to the memory device 16a, it then provides a command word to a command decoder and sequencer 60. The command decoder and sequencer 60 generates a large number of internal control signals to control the operation of the memory device 16a during a memory transfer.
The address capture circuit 48 also receives the command words from the command bus 50 and outputs a 20-bit address corresponding to the address information in the command packet. The address is provided to an address sequencer 64 which generates a corresponding 3-bit bank address on bus 66, a 10-bit row address on bus 68, and a 7-bit column address on bus 70. The column address and row address are processed by column and row address paths 73, 75 as will be described below.
One of the problems of conventional DRAMs is their relatively low speed resulting from the time required to precharge and equilibrate circuitry in the DRAM array. The packetized DRAM 16a shown in FIG. 2 largely avoids this problem by using a plurality of memory banks 80, in this case eight memory banks 80a-h. After a memory read from one bank 80a, the bank 80a can be precharged while the remaining banks 80b-h are being accessed. Each of the memory banks 80a-h receive a row address from a respective row latch/decoder/driver 82a-h. All of the row latch/decoder/drivers 82a-h receive the same row address from a predecoder 84 which, in turn, receives a row address from either a row address register 86, redundant row circuit 87, or a refresh counter 88 as determined by a multiplexer 90. However, only one of the row latch/decoder/drivers 82a-h is active at any one time as determined by bank control logic 94 as a function of a bank address from a bank address register 96.
The column address on bus 70 is applied through a column address path 75 to a redundant column circuit 71 that determines if the column address corresponds to a defective address. The redundant column circuit 71 outputs either the column address or a redundant column address to a column latch/decoder 100 which supplies I/O gating signals to an I/O gating circuit 102. The I/O gating circuit 102 interfaces with columns of the memory banks 80a-h through sense amplifiers 104. Data is coupled to or from the memory banks 80a-h through the sense amplifiers 104 and I/O gating circuit 102 to a data path subsystem 108 which includes a read data path 110 and a write data path 112. The read data path 110 includes a bank of DC sense amplifiers 103 and a read latch 120 that amplify and store data from the I/O gating circuit 102. In the memory device 16a shown in FIG. 2, 64 bits of data are stored in the read latch 120. The read latch then provides four 16-bit data words to an output multiplexer 122 that sequentially supplies each of the 16-bit data words to a read FIFO buffer 124. Successive 16-bit data words are clocked through the read FIFO buffer 124 by a clock signal RCLK generated from the internal clock CKINT by a programmable delay circuit 126. The read FIFO buffer 124 sequentially applies the 16-bit words to a driver circuit 128 which, in turn, applies the 16-bit data words to a data bus 130 forming part of the processor bus 14.
The write data path 112 includes a receiver buffer 140 coupled to the data bus 130. The receiver buffer 140 sequentially applies 16-bit words from the data bus 130 to four input registers 142, each of which is selectively enabled by a signal from a clock generator circuit 144 responsive to the data clock DCLK applied to the memory device 16a on line 132. As with the external clock signal CKEXT and command packet CA0-CA9, the memory controller or other device (not shown) normally transmits the data to the memory device 16a in synchronism with the data clock signal DCLK. Thus, the input registers 142 sequentially store four 16-bit data words and combine them into one 64-bit data word applied to a write FIFO buffer 148. The write FIFO buffer 148 is clocked by a signal from the clock generator 144 and an internal write clock WCLK to sequentially apply 64-bit write data to a write latch and driver 150. The write latch and driver 150 applies the 64-bit write data to one of the memory banks 80a-h through the I/O gating circuit 102 and the sense amplifiers 104.
As mentioned above, an important goal of the SyncLink architecture is to allow data transfer between a processor and a memory device to occur at a significantly faster rate. However, as the rate of data transfer increases, it becomes more difficult to maintain synchronization between signals transmitted to the memory device 16a. For example, as mentioned above, the command packet CA0-CA9 is normally transmitted to the memory device 16a in synchronism with the external clock signal CKEXT, and the data is normally transmitted to the memory device 16a in synchronism with the data clock signal DCLK. However, because of unequal signal delays and other factors, the command packet CA0-CA9 may not arrive at the memory device 16a in synchronism with the external clock signal CKEXT, and the data may not arrive at the memory device 16a in synchronism with the data clock signal DCLK. Even if these signals are actually coupled to the memory device 16a in synchronism with each other, they may loose synchronism once they are coupled to circuits within the memory device. For example, internal signals require time to propagate to various circuitry in the memory device 16a. Difference in length of the signal routing can cause differences in the times at which signals reach the circuitry. Differences in capacitive loading of signal lines can also cause differences in the times at which signals reach the circuitry. These differences in arrival times can become significant at high speeds of operation and eventually limit the operating speed of memory devices.
As mentioned above, the above-described problems are exacerbated as timing tolerances become more severe with higher data transfer rates. For example, if the internal clock CKINT derived from the external clock CKEXT does not latch the command packet CA0-CA9 at the proper time, errors in the operation of the memory device may result. Similarly, if the data clock DCLK does not latch the data applied to the memory device at the proper time, data errors may result.
Unfortunately, there has heretofore been no suitable means for ensuring that digital signals, such the command packets CA0-CA9 and digital signals, are latched in memory devices, such as the packetized DARM shown in FIG. 2, at the proper time at very high data transfer rates.
Although the foregoing discussion is directed to the need for faster command buffers in packetized DRAMs, similar problems exist in other memory devices, such as asynchronous DRAMs and synchronous DRAMs, which must process control and other signals at a high rate of speed.